Thursday, August 22, 2013

More Neutrino Data From Daya Bay

Daya Bay's first results were announced in March 2012 and established the unexpectedly large value of the mixing angle theta one-three, the last of three long-sought neutrino mixing angles. The new results from Daya Bay put the precise number for that mixing angle at sin2(13)=0.090 plus or minus 0.009.. . .

From the KamLAND experiment in Japan, they already know that the difference, or "split," between two of the three mass states is small. They believe, based on the MINOS experiment at Fermilab, that the third state is at least five times smaller or five times larger. Daya Bay scientists have now measured the magnitude of that mass splitting,  |Δm2ee|, to be (2.59±0.20)x10-3 eV2. The result establishes that the electron neutrino has all three mass states and is consistent with that from muon neutrinos measured by MINOS. Precision measurement of the energy dependence should further the goal of establishing a "hierarchy," or ranking, of the three mass states for each neutrino flavor.

MINOS, and the Super-K and T2K experiments in Japan, have previously determined the complementary effective mass splitting (Δm2μμ) using muon neutrinos. Precise measurement of these two effective mass splittings would allow calculations of the two mass-squared differences (Δm232 and Δm231) among the three mass states. KamLAND and solar neutrino experiments have previously measured the mass-squared difference Δm221 by observing the disappearance of electron antineutrinos from reactors about 100 miles from the detector and the disappearance of neutrinos from the sun.
From this press release.

Neither of the two numbers is far from previous estimates. As of March 2012, the estimates were sin2(2θ13) = 0.092±0.017 and |Δm231| ≈ |Δm232| ≡ Δm2 atm = 2.43+0.13−0.13×10−3 eV2.

The precision in the theta13 number is about twice as great as the estimate from a year and a half ago, and is slightly lower than previously estimated. But, the results are consistent with each other at the one sigma level. The mass splitting estimate is consistent with prior data and similar in precision on a percentage basis.


andrew said...

A September 26, 2013 summary is also here and is similarly consistent.

andrew said...

A post from April 2013 summary with PDG data.

andrew said...

"The result I read in paper sixteen was Neff=3.30 +/- 0.27 v. Neff 3.046 for the three Standard Model neutrinos. So, their result is a little less than one sigma from the Standard Model value. A four neutrino model would have an Neff of a bit more than 4.05, which is about three sigma from the measured value which is roughly a 99% exclusion and is a confirmation of the Standard Model.

Planck also combines data from multiple sources puts a cap on the sum of three neutrino masses in a three Standard Model neutrino scenario of 0.24 eV (at 95% CI) with a best fit value of 0.06 eV. The floor from non-astronomy experiments is 0.06 eV in a normal neutrino mass hierachy (based on the difference between mass one and mass two, and between mass two and mass three which are both known to about two significant digits) and 0.1 eV in an inverted neutrino mass hierachy. In a normal neutrino mass hierarchy, this puts the mass of the electron neutrino at between 0 and 0.06 eV, with the low end preferred (I personally expect that an electron neutrino is significantly less than the mass difference between the first and second neutrino type of about 0.006 eV).

Note that a particle that is in the hundreds or thousands of eVs would not count towards Neff because it is not light enough to be relativistic at 380,000 years after the Big Bang. So, it really only rules out a light sterile neutrino, rather than a heavy one. The LSND and MiniBooNE reactor anomalies have hinted at a possible fourth generation sterile-ish neutrino of about 1.3 eV +/- about 30%, so the Planck people did a study on the sum of mass limits if there were a disfavored four and not just three relativistic species and came up with a cap on sterile neutrino mass in that scenario of about 0.5 eV +/- 0.1 eV, which is about 2.5 sigma away from the value of the LSND/MiniBooNE anomaly estimates considering the combined uncertainties.

LEP ruled out a fourth species of fertile neutrino of under 45 GeV, and I wouldn’t be going out on a limb to say without actually doing the calculations that a fertile neutrino of 45 GeV to 63 GeV, if it existed, would have wildly thrown off all of the Higgs boson decay cross-sections observed (since a decay to a 45 GeV to 63 GeV neutrino-antineutrino pair from a 125.7 GeV Higgs boson would have been a strongly favored decay path if it existed) and is in fact therefore excluded by the lastest round of LHC data."

From here.

andrew said...

In a "normal" neutrino mass hierarchy, an example of a set of masses that would fit the current data would be roughly:

Mass State 1: 1 meV/c^2
Mass State 2: 9 meV/c^2
Mass State 3: 50 meV/c^2
Total number of mass states: 3
Combined mass of three states: 60 meV/c^2

In a normal hierarchy, there is more room (on a percentage basis) for downward uncertainty in mass state 1 than there is for mass state 1 on the upside or in the estimates of mass states 2 and 3. Mass state three is 20,000 to 40,000 times lighter than the preferred 2 keV mass estimate for warm dark matter particles.